Measurement of $CP$ Violation in $B^0\to \psi(\to\ell^+\ell^-)K^0_\mathrm{S}(\to \pi^+\pi^-)$ Decays

A measurement of time-dependent $CP$ violation in the decays of $B^0$ and $\bar{B}^0$ mesons to the final states $J/\psi (\to \mu^+ \mu^-) K^0_S$, $\psi(2S) (\to \mu^+ \mu^-) K^0_S$ and $J/\psi (\to e^+ e^-) K^0_S$ with $K^0_S \to \pi^+ \pi^-$ is presented. The data correspond to an integrated luminosity of 6 fb$^{-1}$ collected at a center-of-mass energy of $\sqrt{s} = 13 \, \text{TeV}$ with the LHCb detector. The $CP$-violation parameters are measured to be $S_{\psi K^0_S} = 0.717 \pm 0.013 \, \text{(stat)} \pm 0.008 \, \text{(syst)}$ and $C_{\psi K^0_S} = 0.008 \pm 0.012 \, \text{(stat)} \pm 0.003 \, \text{(syst)}$. This measurement of $S_{\psi K^0_S}$ represents the most precise single measurement of the CKM angle $\beta$ to date and is more precise than the current world average. In addition, measurements of the $CP$-violation parameters of the individual channels are reported and a combination with the LHCb Run 1 measurements is performed.